Detect a Pi 5 for its kernel instead of a per-host flag

`pi_standard_setup_install_pi5_kernel` now defaults to "auto": the Pi 5 kernel is
installed when the device tree says Raspberry Pi 5, which covers a card moved over from
a Pi 4 and is a no-op on current images that ship it. true/false still force it, and
musicdolphin no longer needs its own override.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

roles/pi_standard_setup/defaults/main.yml

@@ -11,6 +11,7 @@ ansible_become_password: "raspberry"
 ansible_become: true
 
 # Install the Pi 5 (BCM2712) kernel, kernel_2712.img (an arm64 package, hence the :arm64 on
-# the armhf userland). A card imaged on a Pi 4 does not have
-# it, and a Pi 5 will not boot without. Harmless on a Pi 4, which keeps its own kernel.
-pi_standard_setup_install_pi5_kernel: false
+# an armhf userland). A card imaged on a Pi 4 does not have it, and a Pi 5 will not boot
+# without. "auto" installs it when the detected model is a Pi 5 (a no-op on current
+# images, which ship it); set true/false to force.
+pi_standard_setup_install_pi5_kernel: auto

roles/pi_standard_setup/tasks/main.yml

@@ -116,11 +116,17 @@
     cache_valid_time: 7200
     state: present
 - name: Install the Pi 5 kernel
+  # Only where it can matter: a Pi 5 running an image that was built for an older Pi
+  # (a card moved over from a Pi 4). A current arm64 image already ships it, and then
+  # this is a no-op; an armhf userland needs the arm64 kernel package cross-installed.
   ansible.builtin.apt:
     name: "linux-image-rpi-2712:arm64"
     cache_valid_time: 7200
     state: present
-  when: pi_standard_setup_install_pi5_kernel
+  when: >-
+    (pi_standard_setup_install_pi5_kernel | string | lower == 'true')
+    or (pi_standard_setup_install_pi5_kernel | string | lower == 'auto'
+        and 'Raspberry Pi 5' in (pi_standard_setup_raspberry_model.content | b64decode))
   notify: Reboot after standard setup
 - name: Copy vim config
   ansible.builtin.copy: